Device for simulating longitudinal and transverse accelerations on a vehicle compartment

ABSTRACT

The device for simulating longitudinal and transverse accelerations on a vehicle compartment has a first motion level for longitudinal displacement of the compartment and, placed on top of the former, a second motion level for transverse displacement of the compartment. To simplify the construction in the interests of low production costs and to allow large mechanical loads to be applied while keeping the height of the assembly low, a pair of bogies configured at a fixed longitudinal distance from one another can run in each level on the appropriate one of two parallel tracks. Each track of the mutually perpendicular pairs is at least in the front and rear track regions uniformly ascending. The second motion level directly accommodates the vehicle compartment.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for simulating longitudinal andtransverse accelerations on a vehicle compartment of a vehicle, having afirst motion level for longitudinal displacement of the compartment inwhich a pair of bogie wagons can travel on one of two parallel-spacedrails where the rails ascend uniformly, at least in the front and rearregions of the rail ends, and are symmetric to the center of the railsand a second motion level which is located above the first motion levelfor transverse displacement of the compartment and in which a secondpair of bogie wagons can travel on one of two parallel-spaced rails thatextend perpendicular to the rails of the first motion level.

In a known motion simulator of this type for a high-speed rail vehicle,the pair of rails associated with the first plane of motion is disposedon a base frame with two ramps that ascend from the center toward theedge at the same angle of ascension, with each rail of a rail pair beingdivided into two rail segments, each of which is secured to one of theramps. The four bogie wagons respectively located on one of the fourrail segments support an intermediate frame, on the upper side of whichthe rail pair associated with the second plane of motion is secured in alevel plane. The four bogie wagons that run on the second rail pairsupport a platform that receives the vehicle compartment with theinterposing of a tipping mechanism for inclining the vehicle compartmentlaterally about a horizontal axis oriented transversely to the railsextending on the intermediate frame. With corresponding control of thedrive motors for the intermediate frame and the platform, as well as thetipping mechanism, the vehicle compartment can be moved for simulatinglongitudinal and transverse accelerations with an inclination of thevehicle in the longitudinal and transverse directions. This type ofapparatus for motion simulation is very costly to build, and has arelatively large structural height as dictated by the presence of baseframes, intermediate frames, the platform and the tipping mechanism.

It is the object of the invention to provide an apparatus for motionsimulation of the type mentioned at the outset that can be manufacturedat low cost because of a simplified technical construction, and canreceive large mechanical loads, such as heavy vehicle compartments andso-called domes for viewing system simulation, and also has a smallstructural height.

In accordance with the invention, the object is accomplished in anapparatus for motion simulation of the generic type defined in thepreamble to claim 1 and having the features of the characterizingportion of claim 1.

SUMMARY OF THE INVENTION

The apparatus of the invention for motion simulation has the advantagethat a lateral, inclined motion of the vehicle compartment is producedby a simple rail guidance, without a tipping mechanism that is costly toproduce. The omission of the tipping mechanism not only lowersproduction costs, but also results in a low construction. Rails andbogie wagons need not be specially manufactured individually, but arecommercially available in diverse dimensions and qualities forreasonable prices. It is possible to stress the apparatus of the typedescribed in the first paragraph of the BACKGROUND OF THE INVENTIONwhere the rails associated with the first motion level are secured to astationary base frame and the rails associated with the second motionlevel are secured to the underside of a platform which faces the baseframe and supports the vehicle compartment, and a movable connectionwith three mutually perpendicular axes of rotation exists between eachof the bogie wagons that travel on the rails of the base frame and abogie wagon that travels on the rails of the platform inventionconsiderably more without reducing or electric-motor-actuated actuatorsof a tipping mechanism. The apparatus of the invention permits a verygood simulation of purely translatory motions with a simultaneoussuperposed inclination, both in the longitudinal and transversedirections, with a structurally simple system that can be put undertremendous stress. This system can be used in extremely faithfulsimulation of sustained accelerations in the longitudinal and transversedirections. Vertical accelerations of the system remain beneath theperceptible limit. The drives can be electrical, hydraulic or pneumatic.

The further claims disclose useful embodiments of the apparatus of theinvention for motion simulation that involve advantageous refinementsand modifications of the invention.

In accordance with a preferred embodiment of the invention, the railsassociated with the first motion plane are secured to a stationary baseframe, and the rails associated with the second motion plane are securedto the underside of a platform that supports the vehicle compartment,which side faces the base frame. A movable connection with three axes ofrotation that are oriented perpendicular to one another and can beembodied with, for example, a ball-and-socket joint exists between eachof the bogie wagons traveling on the rails of the base frame and a bogiewagon traveling on the rails at the platforms. This constructionproduces an extremely low structure that is very sturdy and can handle agreat deal of stress. With the arrangement of the rails and the spacingof the platform bearing points, both the angle of inclination and theplatform design can be varied and adapted to different requirements.With specially-shaped guide rails, the course of the rail curve permitsthe point of sight or the center of the platform to be kept in one planein a horizontal deflection of the platform.

In an alternative embodiment of the invention, the two motion planes canbe realized with rails and bogie wagons in that the bogie wagonsassociated with the first motion plane are secured to a stationary baseframe, and the rails associated with the first motion plane are disposedon the underside of an intermediate frame. The bogie wagons associatedwith the second reference plane are disposed on the top side of theintermediate frame, and the rails associated with the second motionplane are disposed on the underside of a platform that supports thevehicle compartment. The inclusion of the intermediate frame results ina slightly larger structural height of the overall system, but thetranslatory drive for longitudinal and transverse motion of the vehiclecompartment with a superposed inclination is simplified significantly.

In accordance with an advantageous embodiment of the invention, thetranslatory drive in the apparatus is realized in that a spindle drivewhose spindle is driven by an electric motor, and that has a spindle nutthat can be screwed up and down on the spindle, is disposed on the baseframe and the intermediate frame, respectively. The two spindles areoriented perpendicular to one another, and extend through the centerbetween the rails at the intermediate frame or the platform,respectively. The spindle nut is hinged in the center to theintermediate frame or the platform.

The invention is described in detail below in embodiments illustrated inthe drawing. Shown in the following schematic illustrations are:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a side view of an apparatus for simulating motion,

FIG. 2 a view of the apparatus according to arrow II in FIG. 1,

FIG. 3 an exploded view in perspective of the apparatus of FIGS. 1 and2,

FIG. 4 an enlarged representation of the detail IV in FIG. 1,

FIG. 5 a schematic plan view of the apparatus in FIG. 2,

FIG. 6 a view in perspective of a second embodiment of an apparatus forsimulating motion,

FIG. 7 a view of the apparatus according to arrow VII in FIG. 6, and

FIG. 8 a view of the apparatus according to arrow VIII in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus for simulating the motion of a vehicle compartment 10,which is shown in different views and in an exploded view in perspectivein FIGS. 1 through 3, serves to simulate sustained accelerations of thevehicle compartment 10 in the longitudinal and transverse directions. Itpermits a virtually purely translatory motion with a simultaneousinclination of the vehicle compartment 10 in order to utilize the effectof gravity on the driver for simulating sustained accelerations. Thesame applies for the apparatus for motion simulation that is shown, in afurther embodiment, in perspective and in two side views in FIGS. 6through 8. The two apparatuses are constructed according to the samebasic principle, with a first motion level 11 accordingly being providedfor a longitudinal displacement of the compartment and a second motionlevel 12 being provided above the first plane for a transversedisplacement of the compartment. In each motion level 11, 12, two bogiewagons 13 and 14, respectively, are disposed, with fixed longitudinalspacing from one another, to travel on one of two rails 15 and 16disposed with fixed parallel spacing from one another. Starting from thecenter of the rail, each of the rails 15, 16 ascends symmetricallytoward the front and rear rail regions. Each rail 15 and 16 is dividedinto two rail segments 151, 152 or 161, 162, respectively, which extendinclined in opposite directions or with oppositely-oriented slopes andmeet at an obtuse angle in the center of the rail. The vehiclecompartment 10 is received by the second motion level 12. It can befixedly connected to the second motion level 12--as in the embodimentsof FIGS. 1 through 5 and 6 through 8--or supported on this plane by wayof electrical actuators that permit additional pitching and rollingmotions of the vehicle compartment 10, in arbitrary directions, withrespect to the second motion level 12.

In the embodiment of the apparatus for motion simulation according toFIGS. 1 through 5, this described basic principle is modified such thatthe rails 15 of the first motion level 11 are secured to a stationarybase frame 17, and the rails 16 of the second motion plane are securedto a platform 18 that supports the vehicle compartment 10. Correspondingto the two rails 15 disposed with a parallel spacing d (FIG. 3) andhaving the rail segments 151 and 152, the base frame 17 comprises tworamp-like side parts 19, which are connected to one another bytransverse webs 20. Two ramps are embodied on the top side of each sidepart 19, which ramps ascend from the center of the side part 19 towardthe front or rear end of the side part 19; a rail segment 151 or 152 ofthe rails 15 is secured to each ramp. The platform 18 has a front and arear triangular, transverse support 21, with the hypotenuse of thetransverse support 21 pointing toward the top side of the platform 18and the two sides that form an obtuse angle pointing toward theunderside of the platform 18. The two transverse supports 21 are rigidlyconnected to one another by a connecting body 22 (shown in dashed linesin FIG. 1). Corresponding to the triangular shape of the transversesupports 21, the connecting body 22 is pyramidal with a triangularpyramidal cross section, and the horizontally-oriented top side of theconnecting body 22 receives the vehicle compartment 10. A rail segment161 or 162 of the rails 16 associated with the platform 18 is secured tothe two sides of the two transverse supports 21 pointing down toward thebase frame 17. The parallel spacing of the rails 16 is indicated by l inFIG. 3.

Two bogie wagons 13 travel on each rail 15 of the base frame 17, witheach pair of bogie wagons 13 having a fixed spacing from the other thatcorresponds to the spacing l of the rails 16 at the platform 18. One ofthe two bogie wagons 13 runs on the rail segment 151 of the rail 15, andone bogie wagon 13 runs on the rail segment 152 of the rail 15. Theembodiment of a transition region at the meeting point of the two railsegments 151, 152 would allow the bogie wagon 13 to run from the onerail segment 151 or 152 onto the other rail segment 152 or 151, and viceversa. Two identical bogie wagons 14 can travel on each rail 16 of theplatform 18, with the two bogie wagons 14 of a bogie wagon pair beingmaintained with a fixed spacing from one another that corresponds to thespacing d of the two rails 15 of the base frame 17. As can be seen fromFIG. 1, a bogie wagon 14 can travel on a respective rail segment 161;again, the corresponding embodiment of the meeting point of the two railsegments 161, 162 could permit the bogie wagon 14 to change over fromthe rail segment 161 or 162 to the other rail segment 162 or 161. Thecooperation of bogie wagon 13 and rail 15 is shown in detail in FIG. 4.The bogie wagon 13 extends around both sides of the rail 15, and issupported on each side via running bearings 26 disposed in both a groove25 in the bogie wagon and a lateral guide groove 27 in the rail 15.Because of the perpendicular orientation of the rails 15 and the rails16 relative to one another, the bogie wagons 13 and 14 traveling onthese rails 15 and 16 are oriented perpendicular to one another. Asindicated schematically in FIGS. 1 and 2 and shown in detail in FIG. 4,each bogie wagon 13 is connected to a bogie wagon 14 by way of aball-and-socket joint 28. However, a different movable connection can beproduced between the bogie wagons 13 and 14 instead of a ball-and-socketjoint 28, which must permit a movement about three mutuallyperpendicular axes of rotation. Examples of such movable connectionsinclude metal-rubber elements, as well as gas-filled or liquid--filledbellows, in which case the gas or liquid filling absorbs the weight ofthe platform.

To produce a translatory acceleration of the vehicle compartment 10 witha superposed inclination, one of the two bogie wagons 13 traveling onthe rails 15 at the base frame 17 is driven synchronously to both sidesin the longitudinal direction of the rails 15. Furthermore, the platform18 is displaced in both directions along the rails 16 such that a bogiewagon 14 on each rail 16 is coupled to a drive fixed to the platform 18.The translatory motion of the platform 18 is indicated by adouble-headed arrow 29 in FIG. 4.

FIG. 5 schematically shows an embodiment of the drive mechanism. Thepairs of bogie wagons 13 that travel on the rails 15 of the base frame17 and are coupled to one another by way of the ball-and-socket joint28, and the bogie wagon 14 that travels on the rails 16 of the platform18 (FIG. 4), are indicated by 13/14 in FIG. 5. Associated with the baseframe 17 and the platform 18, respectively, is an electric-motoractuation drive in the form of a spindle drive 50 or 50', which has aspindle 51 or 51' that is driven by an electric motor 53 or 53', and aspindle nut 52 or 52' that can be screwed up and down on the spindle 51or 51'. The spindle 51 or 51' is held so as to rotate on the base frame17 or the platform 18, and the spindle nut 52 or 52' is hinged to abogie wagon 13 or 14 on the one rail 15 or 16. A closed cable pull 54 or54' leads from each bogie wagon 13 or 14 on the one rail 15 to one ofthe two bogie wagons 13 or 14 on the other rail 15 or 16. The cable pull54 is guided over deflection rollers 55, 56, 57 and 58 secured to thebase frame 17, while the cable pull 54' is guided over deflectionrollers 55', 56', 57', and 58' secured to the platform 18. Twodeflection rollers 55, 56 or 57, 58 (correspondingly, 55', 56' or 57',58') are respectively disposed at the end of the paths of displacementof the two bogie wagons 13 (correspondingly, 14), that is, at the endsof the rail segments 151 (correspondingly, 161).

If the electric motor 53 rotates, the spindle 51 is screwed further intoor out of the spindle nut 52, causing the bogie wagon pair 13/14 totravel along the rail 15 in one or the other direction. The cable pull54 displaces the other bogie wagon pair 13/14 by the same path ofdisplacement on the other rail 15. Thus, the entire platform 18 travelsalong the two rails 15.

If the electric motor 53' rotates, the spindle 51' is screwed up or downin the spindle nut 52' at the bogie wagon 14. Hence, the position of thebogie wagon 14 relative to the platform 18 changes, and the cable pull54' also changes the relative position of the bogie wagon 14 on theother rail 16. Because the two bogie wagons 14 are fixed on the tworails 15 at the base frame 17 by their connection to the bogie wagon 13,the entire platform 18 is displaced transversely to the base frame 17.

The further embodiment of the apparatus for motion simulation, which isshown in FIGS. 6 through 8 and operates according to the same basicprinciple, again has a stationary base frame 30 and a platform 31 thatsupports the vehicle compartment 10, only indicated schematically. Incontrast to the apparatus according to FIGS. 1 through 5, an additionalintermediate frame 32 is provided between the base frame 30 and theplatform 31. The base frame 30 comprises two side parts 33 that arespaced from one another and are connected to one another by transversewebs 34. The intermediate frame 32 comprises a triangular, fronttransverse support 35 and an identical, rear transverse support 35,which are connected to one another by longitudinal webs 36. Thetriangular transverse supports 35 are disposed such that theirhypotenuses point up and their two sides meeting at an obtuse anglepoint down toward the base frame 30. The base frame 30 and theintermediate frame 32 define the first motion plane 11 of the system,with the rails 15 associated with this motion plane 11 being secured tothe sides of the transverse supports 35. The rails 15 on each transversesupport 35 are again divided into two rail segments 151 and 152, whichrespectively lie on a cathetus of the transverse support 35. The bogiewagons 13 that travel on the rails 15 are secured to the base frame 30with spacing from one another, specifically two bogie wagons 13 on thetop side of the side parts 33. The spacing between the two bogie wagons13 disposed on a side part 33 is determined by the parallel spacing ofthe two rails 15 at the intermediate frame 32.

The platform 31 is constructed in the same manner as the intermediateframe 32, but its position on the intermediate frame 32 is rotated by90° in the plane. It has two triangular longitudinal supports 37, whichare connected to one another by transverse webs 38. The triangular tipsof the longitudinal supports 37 point toward the intermediate frame 32,and their sides meeting at an obtuse angle support the rails 16 of thesecond motion plane 12, which are again divided into two rail segments161 and 162 and are secured to each cathetus of the longitudinal support37. The bogie wagons 14 of the second motion plane, which travel on therails 16, are secured to the top side of the intermediate frame 32,specifically to the hypotenuse of the triangular transverse supports 35of the intermediate frame 32. The spacing between the two bogie wagons14 disposed on a transverse support 35 of the intermediate frame 32 isdetermined by the spacing between the two rails 16 on the sides of thelongitudinal supports 37 of the platform 31.

The intermediate frame 32 and the platform 31 are displaceable to bothsides in the longitudinal direction of their rails 15 and 16,respectively, which can be effected pneumatically, hydraulically or withan electric motor. In the embodiment of FIGS. 6 through 8, actuatingdrives with electric motors are provided in the form of two spindledrives 40 and 40'. Each spindle drive 40 and 40' includes a spindle 41or 41' that is driven by an electric motor 43 or 43', and a spindle nut42 or 42' that can be screwed up and down on the spindle 41 or 41'. Inthe spindle drive 40 for the intermediate frame 32, the spindle 41 isseated in the base frame 30 and extends in the center between the rails15 at the intermediate frame 32. The spindle nut 42 is secured to alongitudinal web 36 of the intermediate frame 32, which extends from tipto tip of the two triangular transverse supports 35 of the intermediateframe 32. In the spindle drive 40' for the platform 31, the spindle 41'is disposed with an electric motor 43' in the intermediate frame 32,with the spindle 41' extending in the center between the rails 16 at theplatform 31. The spindle nut 42' is disposed at a transverse web 38extending between the tips of the two triangular longitudinal supports37. Depending on the direction of rotation of the electric motors 43,43', the platform 18 is displaced, with its rails 16, in one or theother direction on the bogie wagons 14 at the intermediate frame, andthe intermediate frame 32 is displaced, with its rails 15, in the one orthe other direction on the bogie wagons 13 of the base frame 30.

The invention is not limited to the above-described embodiments. Thus,the rail segments 151 and 152 or 161 and 162 of equal length need notconverge as straight lines to an obtuse angle. The rails 15, 16 can alsobe bent or possess a special curve that is adapted to specificconditions. In the embodiment of the apparatus according to FIGS. 1through 3, for example, the rail segments 151 and 152 of equal length atthe base frame 17 can extend with opposed inclinations and with paralleloffset, and--as indicated by dashed lines in FIG. 2--can cross at anobtuse angle. This crossing permits a greater horizontal deflection anda larger angle of inclination with the same dimensions of the platform18.

The bogie wagons 13, 14 can, of course also be replaced by other rollingelements, and the rails can be replaced by roller tracks that cooperatewith the rolling elements.

The two rails 15 at the base frame 17 and the two rails 16 at theplatform 18 having the rail segments 151 and 152 and 161 and 162,respectively, can also be replaced by a pair of rails extending withparallel spacing from one another. A bogie wagon 13 or 14 then travelson each rail segment of the rail pair, with a total of four bogie wagons13 or 14 for each rail pair. The two bogie wagons 13 and 14 traveling onparallel rail segments of the rail pair are connected by a connectingplate to form a bogie wagon pair. The ball-and-socket joints 28 aredisposed between the connecting plates of two bogie wagon pairsassociated with the base frame 17 and the platform 18.

I claim:
 1. An apparatus for simulating longitudinal and transverseaccelerations on a vehicle compartment of a vehicle, said apparatuscomprising:a first motion level for longitudinal displacement of thecompartment having a pair of first rails with fixed parallel spacing,said first rails ascending uniformly, at least in the front and rearregions of the first rail ends, and symmetrically from the center of therails; a stationary base frame, said first rails being secured to saidstationary base frame; a pair of longitudinal bogie wagons disposed onone of said first rails with fixed longitudinal spacing there between sothat said longitudinal bogie wagons travel on said one of the pair offirst rails; a second motion level, which is located above the firstmotion level, for a transverse displacement of the compartment having apair of second rails, said second rails having a fixed parallel spacingthere between, extending perpendicular to said first rails in the firstmotion plane, and ascending uniformly, at least in the front and rearregions of the second rail ends, and symmetrically from the center ofthe second rails; a platform having an underside and supporting saidvehicle compartment, the underside of said platform facing said baseframe, said second rails being secured to said underside of theplatform; a pair of traverse bogie wagons disposed on one of said pairof second rails with fixed longitudinal spacing there between so thatsaid traverse bogie wagons travel on said one of the second rails; and amovable connection with three mutually perpendicular axes of rotationprovided between each of said longitudinal bogie wagons that travel onsaid first rail and an associated traverse bogie wagon that travels onsaid second rail of the platform.
 2. The apparatus according to claim 1,wherein said movable connection is produced by a ball-and-socket joint.3. The apparatus according to claim 1, further comprising an actuatingdrive which acts on one of a longitudinal or a traverse bogie wagon thattravels on one of the one of the first and second rails at the baseframe and the platform, respectively, so as to synchronize thedisplacement of the longitudinal and traverse bogie wagons.
 4. Theapparatus according to claim 3, wherein said drive is secured to boththe base frame and the platform, said drive engaging one of alongitudinal or a traverse bogie wagon, and the other one of thelongitudinal or traverse bogie wagon being coupled to the engaged bogiewagon for performing synchronous motion.
 5. The apparatus according toclaim 4, wherein said drive is controlled by an electric motor and isconfigured as a spindle drive having a spindle that is actuated by theelectric motor and is seated at either the base frame or the platform,and a spindle nut that is hinged to either the longitudinal or traversebogie wagon and is screwed up and down on the spindle.
 6. The apparatusaccording to claim 4, wherein the synchronous coupling of the otherbogie wagon at the base frame or the platform is effected by a closedcable pull that is guided over deflection rollers.
 7. An apparatus forsimulating longitudinal and transverse accelerations on a vehiclecompartment of a vehicle, said apparatus comprising:a first motion planefor longitudinal displacement of the compartment having a pair of firstrails with fixed parallel spacing, said first rails ascending uniformly,at least in the front and rear regions of the first rail ends, andsymmetrically from the center of the rails; a stationary base frame; anintermediate frame having an underside and a top side, said first railsbeing disposed on the underside of said intermediate frame, a pair oflongitudinal bogie wagons associated with said first motion plane aresecured to said stationary base frame and travel on one of said firstrails with fixed longitudinal spacing there between; a second motionplane, which is located above the first motion plane, for a transversedisplacement of the compartment having a pair of second rails, saidsecond rails being secured to the top side of said intermediate frameand having a fixed parallel spacing there between, said second railsextending perpendicular to said first rails in the first motion level,and ascending uniformly, at least in the front and rear regions of thesecond rail ends, and symmetrically from the center of the second rails;a platform having an underside and supporting said vehicle compartment,the underside of said platform facing said intermediate frame, saidsecond rails being secured to said underside of the platform; and a pairof traverse bogie wagons secured to the top side of said intermediateframe travel on one of said pair of second rails with fixed longitudinalspacing there between.
 8. The apparatus according to claim 7, whereinthe intermediate frame and the platform are displaceable to both sidesin the longitudinal direction of their respective first and secondrails.
 9. The apparatus according to claim 6, further comprising aspindle drive respectively disposed at the base frame and at theintermediate frame, each drive having a spindle that is driven by anelectric motor, and a spindle nut which screws up and down on thespindle, said spindles of each drive extend perpendicular to one anotherand are disposed in the center between the respective first or secondrails, said spindle nuts being hinged in the center to the respectiveintermediate frame or the platform.
 10. The apparatus according to claim1, wherein each first rail forms an obtuse angle in its center and eachsecond rail forms an obtuse angle in its center such that each first andsecond rail form rail segments of equal length and that meet at theobtuse angle.
 11. The apparatus according to claim 1, wherein each firstand second rail is curved in an arc shape.
 12. The apparatus accordingto claim 1, wherein each of the first rails secured to the base framecomprises two rail segments that extend with opposing inclinations andwith parallel offset, and meet at an obtuse angle.
 13. The apparatusaccording to claim 1, wherein said movable connection is produced by acardan joint that can rotate about the height axis.
 14. The apparatusaccording to claim 1, wherein said movable connection is produced by ametal-rubber element.
 15. The apparatus according to claim 1, whereinsaid movable connection is produced by a gas or liquid filled bellows.16. The apparatus according to claim 1, wherein the actuating drive iscontrolled pneumatically.
 17. The apparatus according to claim 1,wherein the actuating drive is controlled hydraulically.
 18. Theapparatus according to claim 8, wherein the intermediate frame and theplatform are displaceable by pneumatic or hydraulic means.
 19. Theapparatus according to claim 7, wherein each first rail forms an obtuseangle in its center and each second rail forms an obtuse angle in itscenter such that each first and second rail form rail segments of equallength and that meet at the obtuse angle.
 20. The apparatus according toclaim 7, wherein each first and second rail is curved in an arc shape.